1. Field of the Invention
This invention relates to a protection for overcurrent in a power semiconductor device using semiconductor switching elements such as IGBT.
2. Related Art
FIG. 4 shows a partial configuration of a power module in a prior art using a power semiconductor switching element such as IGBT. On/off operation of a switching element 31 such as IGBT is controlled by a gate drive circuit 33. In the conventional power module, a resistor 32 is disposed serially on common (emitter) side of the switching element 31 for overcurrent protection. Voltage drop across the resister 32 generated by a main current flowing through the switching element 31 is fed back to the gate driver circuit 33. Output voltage of the gate driver circuit 33 is kept constant, and therefore the voltage between the emitter and the gate substantially drops by the feedback voltage. Thus, the main current at a short-circuit of the switching element 31 is suppressed, and therefore performance at the short-circuit can be improved.
In the above described method, it needs to step down a voltage between the emitter and the gate by a several voltages to suppress the excess main current at the short-circuit. Therefore, a resistance value of the resistor 32 has normally to be set to few score milliohms. Such a few score milliohms of the resistor increases a loss generated in the switching element on the resistor 32 at normal operation. The loss may be too large to be ignored.
This invention is directed to solve the above problems and a purpose of which is to provide a power semiconductor device which can reduce the power loss of the power module at normal operation and achieve overcurrent protection with high precision.
A power semiconductor device according to the present invention comprises a power switching element having two main electrodes and one control electrode, a metal electrode connected to one of main electrodes of the power switching element, and a protection circuit for controlling an operation of the power switching element so that a main current flowing between the main electrodes of the power switching element is detected and the main current is limited when the detected main current is determined to be an overcurrent. The protection circuit detects the main current flowing through the power switching element by detecting a voltage between predetermined two points of the metal electrode. Thus using the metal electrode connected to the power switching element the main current flowing through the switching element is detected, and therefore it is possible to detect an overcurrent stably without variations and reduce the loss generated at the normal operation.
In the power semiconductor device, the metal electrode may be U-shaped. The U-shape reduces inductance of the metal electrode to suppress a noise generated by a change in current due to the switching operation. This can prevent the malfunction at the normal operation without a noise generated on the feedback voltage to the control circuit. Further a resistance of the metal electrode can be lessened to reduce the loss.
The power semiconductor device may comprise a power conversion section for converting a DC voltage into a predetermined three-phase AC voltage for driving a motor, the power conversion section including half-bridge circuits each provided for each phase and comprising the serial connected power switching elements. Then, in each phase, the metal electrode may be connected to one of main electrodes of the power switching element connected on the lower side of the half-bridge. This configuration enables an overcurrent in each phase to be detected.
According to the present invention, the power semiconductor device can be achieved which can reduce a loss generated at the switching operation and can detect an overcurrent stably.